In-line heaters for heating circulating liquid, in particular water, using thick-film technology have been known for a number of years.
EP 0 485 211 discloses a heater for a water heater, shower, washing machine, dishwasher or kettle. The heater includes a vessel for heating liquid, and an electric heating element which is arranged to heat a portion of the vessel. The heating element incorporates a thick-film resistive heating circuit with a thermal fuse included in the thick-film. The document further discloses a triac-type power regulator mounted directly on the heating element that acts as a heat sinker for this triac. Also disclosed is the presence of a thermistor, a temperature sensor, formed on the thick film, a thermal fuse, a flow control valve to continuously adjust the flow rate through the heater, a flow control and a temperature control. These electrical components are connected to a control unit that can be remote or formed as part of the dielectric layer of the thick film at a location close to the inlet pipe where the heater's metal substrate is kept cool by incoming cold water.
DE 197 32 414 discloses a heater that has a metal through-flow passage with an inlet and an outlet and at least one thick film heating element for heating water that circulates in the passage. The thick film heating element extends between the inlet and outlet on the outer surface of the heater. The thick film heating element has a tapered cross-section continuously decreasing in size from the inlet to the outlet. The decreasing section of the thick film and the thus resulting decreasing heat transfer along the passage inhibits accumulation of scale at the end of the passage. The heater may include temperature sensors near the inlet or outlet in the form of thick-film elements. The same idea is disclosed in DE 103 22 034 which concerns a heater having a water circulation passage extending between an outer shell and an inner tube, the shell and/or the inner tube is covered with a thick-film heating element. The water circulation passage may be delimited by a helicoidal fin. The section of the water circulation passage and/or the heating power of the thick-film decrease along the direction of flow. The decreasing heat transfer along the tube is provided to avoid evaporation of water in the heater. The heater may incorporate at the shell or inner tube temperature sensors of the NTC or PTC type in thick-film technology. Another heater with a helicoidal heating conduit is disclosed in DE 197 37 694.
The use of thick-film and other resistive heater technology is also known for hot beverage preparation machines.
U.S. Pat. No. 5,943,472 discloses a water circulation system between a water reservoir and a hot water or vapor distribution chamber of an espresso machine. The circulation system includes a valve, metallic heating tube and pump that are connected together and to the reservoir via different silicone hoses, which are joined using clamping collars.
WO 01/54551 (in the name of the applicant) concerns a liquid heating module for use in a hot beverage machine, which comprises a hollow tube of metallic material and a cylindrical insert located inside the hollow tube. The module incorporates an electrical thick-film resistor on a first part of the outside of the tube for preheating liquid and another electrical resistor on a second part of the outside of the tube for adjusting the temperature of the preheated liquid flowing through the tube. A further electrical resistor for measuring the temperature is incorporated at the module's inlet or outlet. In an embodiment, the heating module is supplied with water via a pump and is connected at its outlet to a conduit for circulating heated water to a coffee extraction chamber.
WO 2004/006742 (in the name of the applicant) discloses a further tubular heating device for beverage preparation machines having a plurality of thick-film resistors that can be empowered in various configurations to adjust the heating. The heating device has an outer hollow metallic tube and an insert made of plastic, metallic or ceramic material. The insert has helicoidal grooves for guiding and circulating water between the outer tube and the insert. The insert may be hollow and may be used for a reversed flow of part of the hot water. U.S. Pat. No. 7,286,752 discloses a similar thick-film tubular heater with an internal helicoidal water circulation conduit.